1. Field of the Invention
The invention relates to a method of coating workpieces of a plastic material with a metallic layer. The coating is precipitated onto the workpiece of a plastic material by means of the well-known cathode sputtering method in a vacuum chamber. Such coatings specifically find application for the production of optical data storage disks. To provide information on such disks for instance magneto-optical or phase change recording processes are used where the information is stored in the coating layer itself or in other optical recording processes, the information is embossed in the disk body underlying the coating by an injection moulding process in which the disk body is then coated with a highly reflective coating so that a laser beam can accordingly scan the information. For cost reasons the disk body is nowadays made of a plastic material. Coating of such plastic material must, however, proceed in such a manner that the coating layer bonds with a sufficiently high adherent strength on the disk body and the coating method does not damage the disk body, especially if provided with said information. With regard to these demands plastic materials are specifically difficult to coat. Such plastic materials, e.g. PMMA, are today commonly used for the production of e.g. laser video disks (LD).
2. Description of the Prior Art
Various treatment processes for increasing the environmental stability and the adhesion of sputter deposited layers on such plastic bodies are known. The environmental stability is usually tested by exposing the coated piece to elevated temperature and humidity conditions, as exemplified below.
Treatment of the surface of the plastic material by means of a plasma is known e.g. from the lecture of G. Legeay, "Surface Modification of natural or synthetic polymers by cold Plasmas", held at the 2nd Conf. Plasma Chem. Tech. 1984. Thereby the surface of a plastic material body is modified by the plasma treatment in such a manner that a subsequently deposited layer adheres better, Gases used for the plasma generation are for instance gases containing oxygen, helium and carbon. The use of helium or oxygen plasma discharges for a treatment of plastic surfaces is also known by the technical writing of J. Hall from: J. Appl. Polym. Sci. Vol. 13, 1969. The object of the plasma treatment is thereby to allow an improved adhesive bonding property to plastic materials. A further technical article of J. Hollahan from: J. Appl. Polym. Sci. Vol. 13, 1969, discloses to subject plastic material surface to a plasma treatment with gas containing ammonia, nitrogen and hydrogen in order to improve the surface adherence property and the wetting capability. For this application use is made of a high frequency plasma discharge.
The U.S. Pat. No. 4,957,603 discloses a coating method for optical storage disks of PMMA. This specification mentions that PMMA (Polymethylmetalcrylate) plastic materials are specifically difficult to be coated by cathode sputtering.
Thereby it is recognized that cathode sputtering would be more amenable to automatic mass production than vapor deposition, but that the problems associated with the environmental stability and adhesion of the sputter coated substrate body would vitiate using such cathode sputtering.
The author of said patent has noted a considerable improvement in the adhesion of a sputter coated metal layer on PMMA when creating an interface polymer which apparently reduces adhesion loss between a sputter deposited metal layer and the bulk of PMMA substrate. It is proposed that before sputter depositing the metal layer, the bulk surface is pretreated in a gas atmosphere comprised of a gas mixture containing argon and a carbon containing gas.
Although the author proposes in one embodiment to perform the sputtering operation of nickel itself using a mixed methane and argon sputtering gas, so that the beneficial surface modification and sputter deposition takes place simultaneously, all that reference clearly discloses is, due to the very low deposition rate of the system used for this work, that pretreating of the bulk substrate surface in a pretreating gas mixture atmosphere, before substantial metallic sputter deposition has occurred, is important.
This is in complete agreement with the mechanism believed to take place by the said author, according to which the excess carbon during exposition of the bulk material to a high energy plasma is believed to modify the bulk polymer surface in such a way to enhance adherence of the subsequently sputter deposited metal film.
The described technique has the sole, but serious disadvantage of essentially and practically necessitating a preparatory process step to pretreat the bulk substrate surface in the sense that a maximal rate metal deposition does not take place from the very beginning of the process, due to system speed limitation. The low deposition rate allows the plastic substrate to be exposed to the mixed gas plasma for a length of time that essentially constitutes a pretreatment step. This lengthens the overall processing time for such workpieces.
Multi-step magnetron sputter coating processes of PMMA bodies with a metallic layer are known with other gas mixtures. Thus, e.g. it is known from the German laid open print no. 40 04 116 to first perform the metal layer deposition by short time sputtering in an argon atmosphere, then to continue in a gas atmosphere of argon and helium and lastly to finish the coating process in a pure argon atmosphere.
In the European patent application no. 0 422 323 a PMMA substrate is magnetron sputter coated with an aluminum coating in a gas atmosphere containing helium.
The summarized drawbacks of such known techniques are:
using a pretreating phase at least at a reduced sputter rate for the metal in an argon/methane atmosphere for preparing the surface of the plastic material substrate or body before applying the metal coating by full-rate sputtering lengthens the overall process time; PA1 magnetron sputter coating such plastic material surface in an atmosphere containing helium results in a longer process time, because helium reduces the yield of sputtered material. Additionally such coatings show a relatively low environmental stability.
From J. Vac. Sci. Technol. A, Vol. 10, N04, July/August 1992, S. Schulz et al., "Thin-film development and methods for compact disk and laser disk manufacturing", a cycle time of 13 sec for sputter coating of PMMA-LD is known.